Technical Field of the Invention
The present invention relates in general to micro-optical bench devices, and in particular to the fabrication of optical surfaces within micro-optical bench devices.
Description of Related Art
Micro Electro-Mechanical Systems (MEMS) refers to the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate through microfabrication technology. For example, the microelectronics are typically fabricated using an integrated circuit (IC) process, while the micromechanical components are fabricated using compatible micromachining processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical components. MEMS devices are attractive candidates for use in spectroscopy, profilometry, environmental sensing, refractive index measurements (or material recognition), as well as several other sensor applications, due to their low cost, batch processing ability and compatibility with standard microelectronics. In addition, the small size of MEMS devices facilitates the integration of such MEMS devices into mobile and hand held devices.
In optical applications, MEMS technology may be incorporated into a micro-optical bench device to enable one or more optical elements to be moveably controlled by a MEMS actuator. Among these applications are interferometers, spectrometers, tunable optical cavities, fiber couplers, optical switches, variable optical beam shapers, optical micro scanners, variable optical attenuators, tunable lasers and many other applications in both sensor and telecommunications domains.
Deeply etched micro-optical benches are typically formed using a Deep Reactive Ion Etching (DRIE) process on Silicon On Insulator (SOI) wafers in order to produce micro-optical and MEMS components that are able to process free-space optical beams propagating parallel to the SOI substrate. Both continuous mode DRIE and pulsed-mode DRIE Bosch processes have been used on SOI wafers. The DRIE Bosch process is a cyclic process switching between an etching cycle in which the substrate is etched in a nearly isotropic manner and a passivation cycle in which the etched sidewalls are protected from the further etching in the next etching cycles. Due to the cyclic nature of the process, scallops are typically formed on the sidewalls of the etched trenches. Although continuous mode DRIE processes avoid the presence of scallops, the etching depth achievable using continuous mode DRIE is typically limited due to the deviation of the energetic ions from the straight-line trajectory.
Within micro-optical benches, the DRIE Bosch process may be used to form both high and low aspect ratio trenches/features. High aspect ratio trenches with a narrow width are of particular interest for MEMS inertial sensors and high density capacitors. In addition, high aspect ratio micromirrors with narrow gaps are generally used in micro-optical benches for creating 1-D photonic band gap filters. In these structures, maintaining the deep progress of etching is challenging as it is more arduous for the etchants to diffuse down to the bottom of the trench and for the etching products to diffuse out. In addition, as a result of scattering on the sidewalls of the trench, the number of ions reaching the bottom of the trench is less for deeper trenches, which leads to inefficient removal of the bottom passivation layer in addition to hampering the passivation layer on the sidewalls. These etching challenges may lead to etched trenches with a positive profile, and may consequently set a limit for the achievable etching depth and aspect ratio.
In contrast, free-space micromirrors are relatively widely separated in free-space, and their surface verticality and smoothness quality are typically of more importance than their aspect ratio. The micromirror surface verticality and roughness are normally controlled by optimizing the DRIE Bosch process and by optionally reducing the cycles' time. Techniques for smoothing out the DRIE Bosch process scallops from the resulting surface using oxidation followed by oxide etching or using short anisotropic wet etching have been proposed. In addition, a combination of a DRIE continuous etching process together with a DRIE Bosch process has been used to produce smooth mirror surfaces at the top part of the mirror. However, the height of the micromirrors in such deeply-etched micro-optical benches is limited, such that beyond this limit, the verticality of the etched surface deteriorates with a highly negative profile and significantly rough surface.
Therefore, what is needed is a method for fabricating high quality deeply-etched micro optical surfaces within micro-optical bench devices that provides control over verticality, surface roughness, coating and overall profile using an optimized deep etching process.